This invention relates to systems and apparatus for monitoring the state of charge of rechargeable (secondary) batteries, and in particular, to metal-gas systems for monitoring the state of charge of nickel-alkaline secondary batteries.
U.S. Pat. No. 4,207,514, issued on June 10, 1980 and assigned to the same asignee hereof discloses a system for monitoring the state of charge of a secondary battery wherein a load charge sensor such as, for example, a shunt resistor, is placed in the current path connecting the secondary battery to a load or a charging supply. The load charge sensor provides signaling for a monitor cell load charge controller which, in turn, controls the magnitude and sense of the current being applied to a metal-gas monitor cell.
In particular, the sense of the current of the monitor cell (i.e., whether it is charging or discharging) is made the same as the sense of the current of the secondary battery, while the magnitude of the monitor cell current is made proportional to the magnitude of the battery current. Since the metal gas cell develops an internal pressure which is proportional to its charging and discharging transferred ampere hours, the resultant pressure in the cell will be indicative of the state of charge of the secondary battery. A state of charge transducer is responsive to the monitor cell pressure and provides a visual indication of the monitor cell pressure and, therefore, the secondary battery state of charge.
The '514 patent also disclosed that heightened reliability and correlation can be achieved when the respective characteristics of the monitor cell and the secondary battery are matched, for example, by employing the same type of solid electrode for each. Thus, for a nickel-alkaline secondary battery, the monitor cell should embody a nickel solid electrode. In this connection, the '514 patent specifically teaches the use of a nickel-oxide hydrogen monitor cell for nickel alkaline batteries, such as, for example, nickel-zinc.
While a nickel-hydrogen monitor cell develops a pressure which is indicative of nickel-alkaline battery state of charge, it has been found that over extended periods of time a gradual error arises between the pressure developed and the state of charge being monitored. It has been recognized by the present applicant that this error occurs because the nickel-hydrogen monitor cell exhibits a self-discharge mechanism which is not characteristic of the self-discharge mechanism in nickel-alkaline batteries. In particular, in the nickel hydrogen cell self-discharge arises as a result of the hydrogen in the cell reacting with the nickel electrode, This, in turn, causes an erroneous state of charge reading over time.
It is therefore an object of the present invention to provide a metal-gas system for monitoring the state of charge of nickel-alkaline batteries wherein the metal-gas system provides more accurate state of charging readings over extended periods of time.